Besides the telltale ocelli of the ubiquitous bimac (and there are a LOT of ocellated octopuses besides bimacs), there are many other subtle keys to IDing octopuses. Finding a pic on the internet that "looks just like" the octopus you saw is only useful if you know specifically what to look for. To ID an unknown ceph, a scientist will be looking at many subtle keys such as particular colorations and patterns, size of the eyes, size and layout of the suckers, length and girth of arms, how far down the arms the web reaches. If the LFS knows (or can at least guess) where the octopus was collected, it's a very useful piece of information.

I don't have a kid, so I'm fuzzy on how big a 3-year olds fist is. Outside of academic circles, most scientists adhere to the International Food Sizing Scale for describing cephalopods size. It is: